Sakura and Shirou being in Imaginary Number Space has some interesting implications.
In Case Files, another magus who uses Imaginary Numbers magecraft preserved their own decapitated head, moments before death, in IN Space, because time doesn't exist in IN Space. This means Shirou and Sakura similarly won't succumb to their wounds no matter how much time passes from everyone else's point of view.
In FGO, being in IN Space protects the survivors of the attack on Chaldea's base from the Alien God's subsequent bleaching of the world, because IN Space is part of the Reverse Side. This means that Shirou and Sakura may also be protected from whatever is going to happen to Miyu's world after all this resolves (probably pruning).
It's not very well explained, but the fact that IN Space is untethered from time and some of the shennanigans we've seen regarding IN Space (in particular, everything about the CCC crossover event in FGO, because the Far Side of the Moon exists in IN Space and BB has Imaginary Number-based abilities) heavily implies that all Parallel Worlds in the Nasuverse are connected to the same Imaginary Number Space. Actually traveling between Parallel Worlds through Imaginary Number Space would probably be nearly impossible for humans, if not for one detail about how the Paper Moon that controls Chaldea's IN Space craft works in FGO. When "resurfacing" from IN Space, it orients itself using "bonds" between the people in the ship and people in Real Number Space.
Finally, assuming everyone else makes it back to Ilya's world safely after all this is resolved, we know of another Imaginary Numbers magus in that world: her world's Sakura.
TL;DR: Hiroyama has given us a way to bring Miyuverse Shirou and Sakura back to Ilyaverse if he wants, assuming they have a way to make sure they don't immediately die when they're back in a place where time exists. Good thing Ilya and Miyu are Holy Grails, huh?